1. Field of the Invention
The present invention relates generally to a downlink Distributed Antenna System (DAS) and, in particular, to a method and apparatus for feeding back downlink channel information in a DAS-based wireless communication system using DAS channel characteristics.
2. Description of the Related Art
A DAS is formed by a Central Processing Unit (CPU) connected to a plurality of transmit antennas distributed within a cell through wired and/or wireless links. Accordingly, an Access Terminal (AT) can establish a virtual cell with a few adjacent distributed antennas.
With the distribution of the transmit antennas over a wide area, a DAS is capable of obtaining macroscopic diversity gain and is advantageous in that it is possible to provide the AT located at a cell boundary with a high data rate, high quality communication service. Further, reduced transmission power of each antenna decreases inter-cell interference, thereby improving system throughput. Accordingly, a DAS is expected to be a core technology for 4th generation wireless communication systems.
When a CPU is connected with M distributed Antenna Ports (APs), each of which uses nT transmit antennas, downlink data generated by the CPU is simultaneously transmitted through total MnT transmit antennas. When multiple reception antennas are used at a receiver, a DAS can efficiently transmit the downlink data with a Multiple-Input and Multiple-Output (MIMO) technique.
In MIMO techniques, a transmitter and a receiver operate with spatial processing based on MIMO Channel State Information (CSI). Particularly in downlink, the transmitter should have the downlink MIMO channel information from nT transmit antennas of a Base Transceiver Station (BTS, hereinafter used with the term “transmitter” interchangeably) to nR receive antennas of the AT (hereinafter, used with the term “receiver” interchangeably).
In a Frequency Division Duplex (FDD) system in which the downlink and uplink are allocated separate frequency bands, the receiver estimates the downlink channel state and feeds back the estimated downlink CSI to the transmitter. However, it is inevitable that the number of transmit antennas increases in the DAS, significantly increasing the amount of feedback of CSI.
Therefore, there is a need for a feedback technique for transmitting the CSI using a limited amount of feedback information, without compromising the increase of the number of the transmit antennas in a DAS.
Further, a channel environment of a DAS significantly differs from a conventional co-located MIMO channel environment. Particularly, because propagation paths between an AT and the AP distributed within a cell are highly different from each other, the average Signal-to-Noise Ratios (SNRs) for the different antennas show significant differences too. For example, the channels from transmit antennas of different APs have uncorrelated channel characteristic, while the channels from the transmit antennas of the same AP to an AT are correlated, i.e., the correlated channel environment and uncorrelated channel environment coexist. Accordingly, if the feedback technique of a conventional MIMO scheme is applied to a DAS without modification, performance is poor. Accordingly, a need exists for a new feedback technique suitable for a DAS channel environment.